Ethanol is the most well-known and commonly used biofuel around the world. It is directly used in blends with gasoline up to 10 volume to volume percent (v/v%) in the US. Blend ratios higher than 15% blends may cause unacceptable corrosion in both blending equipment and consumer cars that are not especially equipped to deal with this biofuel. This limitation is called the “blendwall”. Biodiesel is also a well-known biofuel used as a diesel substitute. Some states in the US already require biodiesel/diesel blends of up to 2% biodiesel. Biodiesel, however, can present engine plugging problems when used at very low temperatures (winter) due to unfavorable cold flow properties. This biofuel can also present storage and stability problems. For example, fatty esters can undergo hydrolysis reactions increasing the acidity of the fuel and, hence, its corrosiveness. Due to the unsaturated nature of the hydrocarbon moieties in biodiesel, this biofuel also presents oxidative instability and bacterial growth can take place in biodiesel during long storage periods. As importantly, biodiesel viability is constrained by the current cost and availability of vegetable oils and animal fats used for its preparation. Due to governmental legislation requiring higher Renewable Fuels Standards (RFS), there is an increasing need for biofuels fungible at high concentrations with current transportation fuels.
Established biofuels, such as ethanol and biodiesel, present serious performance and stability problems. Mono-alcohols with 4-6 carbons are less problematic for fuel blend application since they show less of the corrosion issues associated with ethanol. These alcohols are more hydrophobic than ethanol; hence, they tend to absorb much less water. In addition, C4-C6 mono-alcohols have physicochemical properties closer to gasoline's and, because of their oxygen content; they still serve as octane enhancers. Thus, C4-C6 mono-alcohols will actually make better gasoline blending components than ethanol. However, there is a technology vacuum for the preparation of these alcohols from biomass in high volumes as required for their application as gasoline blending components in high concentrations.
Polyhydric alcohols (compounds with 2 or more hydroxyl groups) can be obtained by subsequent hydrolysis of biomass-derived carbohydrates and hydrogenation of the hydrolysis product. Two main technologies are used for carbohydrate hydrolysis, one technology is hydrothermal-hydrolysis with and without acid catalysis and the second is enzymatic hydrolysis. Enzymatic hydrolysis of carbohydrates produces sugars with high selectivity. Enzymes used for these purpose, however, are cost intensive and highly susceptible to reaction conditions (e.g., pH, temperature, water concentration, chemical inhibition, among other factors), losing their catalytic activity easily. Hydrothermal processing of carbohydrates can produce sugar in high yields, but also has a high tendency to form byproducts, i.e., sugar dehydration products, such as furfurals and levulinic acid. However, even such degradation products of sugars can be converted to polyhydric alcohols with subsequent hydrogenolysis processing. Hence, producing polyhydric alcohols is feasible and can be accomplished with current technology.
Producing mono-alcohols from polyhydric alcohols is challenging. Currently, fermentation of sugars is the main technology used for mono-alcohol synthesis for fuel applications. Fermentation technology has been extensively developed for ethanol production at a commercial scale. Presently, no other mono-alcohols are produced in high volumes from fermentation. Companies, such as BP and DuPont, and academic institutions, have focused their efforts on butanol production through fermentation with some success. Fermentation produces alcohol streams that require a large distillation operation to obtain water-free alcohols and can be energy intensive.
Currently, there is no commercial technology for the production of other fungible biofuels, such as mono-alcohols having between four and six carbon atoms such as butanols, pentanols, and hexanols, in volumes sufficient to compete with currently established biofuels.
There is an increasing desire to use abundantly available cellulosic materials as a source for biofuels.